Control grid arrangement in cathode ray tubes



K. SCHLESINGER 2,185,133

CONTROL'GRID ARRANGEMENT IN QATHODE RAY TUBES Filed Jan. 15, 1937 v 2Sheets-Sheet 1 mas/m Dec. 26, 1939. 2,185,133

CONTROL GRID ARRANGEMENT IN CATHODE RAY TUBES K. SCHLESINGER 2Sheets-Sheet 2 Filed Jan. 15, 1957 Ill/ll Ill Ill/Ar a I V Patented Dec.26, 1939 1 UNITED STATES CONTROL GRID ARRANGEMENT IN CATHODE RAY TUBESKart Schlesinger, Berlin, Germany, assignor, by mesne assignments, toLoewe Radio, 1110., a corporation of New York Application January 15,1937, Serial No. 120,775 in Germany January 20, 1936 9 Claims.

The light intensity control of television cathode ray tubes isperformed, as a rule, by an arrangement in which a circular cathodesurface I is situated behind a perforated diaphragm 2, whilst apositively biassed perforated anode 3 is placed behind the grid 2. Insome arrangements the anode 3 is employed at the same time as adiaphragm, the aperture of which is reproduced sharply on the luminousscreen by a lens mounted in axial consecution thereto. The operation ofsuch an arrangement is unsatisfactory in several respects. Thesensitivities fluctuate within wide limits. The control sensitivitybecomes, as a rule, poorer when the anode potential of 3 increased. Italso deteriorates when the spacing between the grid and the anode isreduced. A decrease of the spacing between the grid and the cathode doesnot necessarily increase the steepness, and moreover. results in anextremely undesirable effect, viz., contraction of the image point inthe case of modulation towards'black. If the aperture in the anode 3 isa diaphragm aperture to be electron-optically reproduced, it isdesirable for the illumination with electrons of this aperture always tocover evenly the entire diaphragm aperture independently of the currentintensity of the ray, so that the size of the image point will remainconstant in respect of all conditions of intensity.

In the following explanation and description of, the present invention,reference will be made to the accompanying drawings each figure of whichshows the system for exciting and con trolling electron emission in acathode ray tube,

Figs. 1, 2a, and 21), being of a diagrammatic type, showing systemswhich are not in accordance with the invention, and in connection withwhich the drawbacks which the invention removes are explained, whereasFigs. 3, 4, and 6, likewise being of a diagrammatic type, and Fig. 5,being a more detailed sectional elevation, show systems which are inaccordance with the invention.

In the arrangement according to Fig. 1, the desired inclependency fromthe beam intensity of the size of the image point is not obtainable, butthe ray progressively contrasts in this arrangement as the intensitydecreases, so that eventually the diaphragm is no more completely filledout by the rays and the size of the image point diminishes steadily asthe intensity is further reduced. In consequence the scanning lines ofthe television image become visible to such an extent as to produce anunpleasant impression.

To explain the above remarks relative to sensitivity and steepness it isassumed that a hot cathode l having a spot of oxide of 1 mm. in diameterdisposed behind the aperture in a grid diaphragm 2 of equal size,requires with a spacing of l between the anode 3 and the grid 2, morethan 30 volts for complete modulation the characteristic having avarying steepness, via, one which decreases as the intensity decreases.Upon increasing the spacing between 2 and 3 to give it an amount of 3mm. the sensitivity is raised to'about 10 volts required for completemodulation, the anode tocathode voltage being in these considerationsassumed to amount to about 500 volts. if the anode to cathode voltage is199. volts instead, the sensitivity is increased still further by afactor of about two or three. It is particularly this effect, viz. theapparent dependency of the steepness on the anode potential, which isquoted in the literature as a general law in support of the usefulnessof a preliminary acceleration. v

The inventor has found that the last mentioned conclusion does notstand. In the following there will be set forth general observationswith regard to the control operation and practical methods of obtaininga light control in televisiontubes, which 1. Result in a considerablesensitivity,

2. Ensure an even thickness of the ray without I tively biassed grid 2and a positively biassed anode 3 is represented by two lines, of whichthe one labelled (1 represents the potentials of the metallic parts,whilst the other b indicates the potential in the axis of the ray, 1.e., within the apertures. The latter potential is less negative in thegrid aperture than the grid itself, since, in accordance with thediagram of the equipotential surfaces given in Fig. 2b the equipotentialsurfaces are bent through the grid aperture. The more positively theanode ii is biassed and the wider the opening in the grid 2 is, the morestrongly will the equipotential surfaces be bent through the gridaperture. occur that at the center-point of the grid aperture thepotentialis zero, (1. e., cathode potential, curve 6 in Fig. 2a)although the grid is biassed to l0 volts (curve coin Fig. 2a). Inconsequence electrons are propagated at the central part of thearrangement, whilst at the marginal zones currentless control is alreadyobtained.

It may accordingly ode l.

The more strongly the equipotential surfaces are bent, the greater willbe the difference between the grid potential and the aperture potential,and the greater will be the contraction of the beam cross section, andaccordingly the smaller the steepness of the modulation characteristic.

According to the invention remedy is obtained by extending the grid inthe direction of the ray, as shown in Fig. 3. This figure shows acathode I mounted behind a control grid 2, the diameter of the openingin which is equal to or greater than the diameter of the imissive layerla on the cath- A second grid 2 is maintained, together with the firstone, at a constant potential. The two grids may also be formed from onesolid piece of material having an axial boring. While now the anode 3 isable to produce a bend of the equipotential surfaces through the gridaperture 2', as indicated by the curve i, the corresponding curvature ofthe equipotential surface 4 through the aperture in the rear grid 2 isin any case much slighter. In consequence the emission of the cathode lis evenly controlled by the grid 2 over its entire cross-section, andthe ray leaves the cathode with a constant cross-section.

In many cases the illumination of the diaphragm aperture of the anode 3with electrons is desired to be as intense as possible in order toobtain as strong a current as possible. For this purpose preliminaryconcentration is required, which should be independent of the raycurrent intensity.

Fig. 4 shows an arrangement having a double grid, as illustrated in Fig.3, and in which the grid on the anode side is made to be conical andthus produces a lens efiect. The anode 3 is a diaphragm having, forexample, a square aperture. The inclination of the grid 2 is preferablymade greater than calculated from geometric considerations by assumingthat the electrons leave the surface 2 at right angles, i. e., the shapeof the grid is so chosen as to result per se in an over-concentration.By varying the bias of the anode 3 an excessive concentration' may beelectrically weakened, and thus regulated.

With arrangements according to Figs. 3 and 4, the emission of thecathode l, irrespective of the amount of the potential of the anode 3,can be completely modulated with 1-3 volts control voltage, if thelength of the grid, i. e., the distance between the front and'rear gridsurfaces, amounts to 2-4 mm. The sizes of the grid apertures have but avery small effect on the sensitivity of the grid. These apertures arepreferably made larger than the emissive surface, viz., preferably oftwice the diameter. Use was made in practice of cathodes of 1 mm. indiameter and grid apertures of 2 mm. in diameter. The distance of thecathode from the first grid has but little effect on the sensitivity.This distance amounts preferably to 1 mm. which value, however, does notrequire to be exactly adhered to.

In practice the provision of a cylinder 5 offers structural advantages,serving for aligning the cathode and the first grid. For preferredstructural details see Fig. 5. According to this figure the cathode ismounted, with the interposition of an insulating tube 6, on the basemember I. A tap incorporating the grid 2 is slipped onto the same basemember. The cylinder 5 is supported by means of two sheet metal collars8 and 9, of which the upper one is at the same time provided with aconical depression'i, which acts as the front grid, having condensinglens properties.

With the aid of the two collars 8 and 9 this control element is locatedin position by means of insulators mon with the anode 3. In this way itis ensured that the preliminarily concentrated ray is directed one tothe aperture of the diaphragm 3 of about mm. diameter exactly centrally.It has been found in practice that an electron lens comprising a conicalelement 2 according to Fig. 5 possesses in itself a good lens eifecteven in respect of rays remote from the axis. It is, however, furtherposible in accordance with the invention to employ the cylindrical lenssystems which, in themselves, have already been proposed before, forexample in an arrangement according to Fig. 6.

In Fig. 6 there is employed a long double grid 2, 2, the two grid facesof which, taken each for themselves, are flat faces. The front grid face2', however, is sunk into the interior of the mount- IU (preferablyglass rods), in coming cylinder 5, so that the edge of this cylinderdiaphragm 2 is sunk into the interior of 5 in.

conjunction with a given potential 3, or vice versa by selection of thepotential 3 in the case of a given depth to which 2 is sunk into 5.

I claim:

1. In and for a cathode ray tube comprising a cathode having an electronemissive spot and an anode mounted in operative relationship thereto forcausing a substantially unidirectional electron emission from saidcathode: a controlling electrode structure mounted between said cathodeand said anode for modulating the emission from said cathode, saidcontrolling electrode structure having an electron entrance aperture andan electron exit aperture both axially aligned with said cathode andsaid anode, said apertures being displaced from each other by a fewmillimetres in the direction from said cathode to said anode, thediameters of said apertures being substantially equal to the diameter ofsaid emissive spot.

2. In and for a cathode ray tube comprising a cathode having an electronemissivespot and anode mounted in operative relationship thereto forcausing a substantially unidirectional electron emission from saidcathode: a controlling electrode structure mounted between said cathodeand said anode for modulating the emission from said cathode, saidcontrolling electrode structure havingv an electron entrance apertureand an electron exit aperture both axially aligned with said cathode andsaid anode, said apertures being displaced from each other by a fewmillimetres in the direction from said cathode to said anode, thediameters of said apertures being equal to about twice the diameter ofsaid emissive spot.

3. In and. for a cathode ray tube comprising a cathode and an anodemounted in operative relationship thereto for causing a substantiallyunidirectional electron emission from said cathode: a controllingelectrode structure mounted between said cathode and said anode formodulating the emission from said cathode, said controlling electrodestructure having an electron entrance aperture and an electron exitaperture both axially aligned with said cathode and said anode, saidapertures being displaced from each other to a,

distance, approximately equal to the diameters of said apertures, in thedirection from said cathode to said anode.

4;. In and for a cathode ray tube comprising a cathode and an anodemounted in operative relationship thereto for causing a substantiallyunidirectional electron emission from said cathode: a cylindersurrounding said cathode and projecting beyond said cathode in thedirection towards said anode, an apertured diaphragm mounted in saidcylinder transverse to the direction from said cathode to said anode,and a further apertured diaphragm of hollow frusto-conical shapeterminating said cylinder atlthe side thereof facing said anode, thehollow side of said further diaphragm facing said anode to cause apreliminary concentration of the electrons emitted from said cathode,said cylinder and diaphragms being adapted to have modulating voltagesrelatively to said cathode impressed thereon for modulating the emissionfrom'said cathode, the apertures in said two diaphragms being axiallyaligned with said cathode and said anode and displaced from each otherin the direction from said cathode to said anode.

5. In and for a cathode ray tube comprising a cathode and an anodemounted in operative relationship thereto for causing a substantiallyunidirectional electron emission irom said cathode: a cylindersurrounding said cathode and projecting beyond said cathode in thedirection towards said anode, and two apertured diaphragms mounted insaid cylinder transverse to the direction from said cathode to saidanode with their apertures axially aligned with said cathode and saidanode and displaced from each other in the direction from said cathodeto said anode, said cylinder projecting in the direction towards saidanode beyond that one of said two diaphragms disposed nearer to saidanode to cause a preliminary concentration of the electrons emitted fromsaid cathode, said cylinder and diaphragms being adapted to havemodulating voltages rela-; tively to said cathode impressed thereon formodulating the'emission from said cathode.

6. In and for a cathode ray tube comprising a cathoe and an anode forcausing a substantially unidirectional electron emission from cathode; acylinder shaped base having said cathode co-axially secured thereto tosupport said said i cathode from the side of said cathode remote fromsaid anode, a hollow cylinder fitted over said base to project beyondsaid cathode in the direction towards said anode, an apertured diaphragmterminating said hollow cylinder at the side thereof facing said anode,a further hollow cylinder fitted over the first said hollow cylinder toproject beyond the first said hollow cylinder in the direction towardssaid anode, a further diaphragm having a central opening and beingsecured to said further hollow cylinder with the mentioned centralopening disposed between the first said diaphragm and the endcross-section of said further hollow cylinder facing said anode, andmeans for locating said further hollow cylinder in position relativelyto said anode.

7. The invention set forth in claim 6, and wherein said furtherdiaphragm, is secured to the edge of said further hollow. cylinderfacing said anode and is of hollow frusto-conical shape, the hollow sideof said further diaphragm facing saidanode to cause a preliminaryconcentration of the electrons emitted from said cathode.

8. The invention set forth in claim 6, and wherein said furtherdiaphragm is fiat and secured to said further hollow cylinder at adistance from the edge thereof facing said anode so that said furtherhollow cylinder projects beyond said further diaphragm in thedirectiontowards said anode to cause a preliminary concentration of the electronsemitted from said cathodev 9. In and for a cathode ray tube comprising acathode and an anode mounted in operative relationship thereto forcausing a substantially uniaxially aligned with said cathode and saidanode,

said apertures heing displaced from each other by a few-millimetres inthe direction from said cathode tosaid anode, said sheets being disposedsubstantially transverse to the direction from said cathode to saidanode and, taken in the mentioned direction, one behind the, other.

KURT SCHLESINGER.

